Genetic differentiation among various populations of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae)

Cryptic Species in Ecotoxicology

The advent of genetic methods has led to the discovery of an increasing number of species that previously could not be distinguished from each other on the basis of morphological characteristics. Even though there has been an exponential growth of publications on cryptic species, such species are rarely considered in ecotoxicology. Thus, the particular question of ecological differentiation and the sensitivity of closely related cryptic species is rarely addressed. Tackling this question, however, is of key importance for evolutionary ecology, conservation biology, and, in particular, regulatory ecotoxicology. At the same time, the use of species with (known or unknown) cryptic diversity might be a reason for the lack of reproducibility of ecotoxicological experiments and implies a false extrapolation of the findings. Our critical review includes a database and literature search through which we investigated how many of the species most frequently used in ecotoxicological assessments show evidence of cryptic diversity. We found a high proportion of reports indicating overlooked species diversity, especially in invertebrates. In terrestrial and aquatic realms, at least 67% and 54% of commonly used species, respectively, were identified as cryptic species complexes. The issue is less prominent in vertebrates, in which we found evidence for cryptic species complexes in 27% of aquatic and 6.7% of terrestrial vertebrates. We further exemplified why different evolutionary histories may significantly determine cryptic species' ecology and sensitivity to pollutants. This in turn may have a major impact on the results of ecotoxicological tests and, consequently, the outcome of environmental risk assessments. Finally, we provide a brief guideline on how to deal practically with cryptic diversity in ecotoxicological studies in general and its implementation in risk assessment procedures in particular. Environ Toxicol Chem 2023;42:1889-1914. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

“Lepidoptera Flies”, but Not Always…Interactions of Caterpillars and Chrysalis with Soil

Lepidoptera, an order of insects traditionally linked to the aerial habitat, are much more diverse in their living environment than the clichéd image we may have of them. The imago stage, which is the most visible in these insects, is not the one that has the most interaction with the environment (usually caterpillars) nor the one that lasts the longest (very often chrysalises). These two stages are often directly related to litter and soil, although only the interaction at the pupal stage seems to follow a phylogenetic logic with two independent evolutionary events for the preference with soil: Use of litter and the upper “O” horizon as protection against predation for the evolutionarily oldest Lepidoptera families, pupation at greater depths (up to 60 centimetres in extreme cases) for the most derived Lepidoptera families; this probably to take advantage of the thermal and moisture buffer provided by the soil. An estimate suggests that about 25% of lepidopteran species worldwide have more or less obligatory interactions with soil.

Fitness comparison of Plutella xylostella on original and marginal hosts using age-stage, two-sex life tables

The diamondback moth, Plutella xylostella, is an important agricultural pest that severely damages cruciferous vegetables. Although previously considered a threat only to Brassica species, P. xylostella has been observed to feed on noncruciferous vegetables. Here, we established a population of P. xylostella on the pea Pisum sativum (PxP population). We compared this PxP population's performance on the pea host plant to a population (PxR) reared on the original host plant radish (Raphanus sativus) for several generations using an age-stage, two-sex life table and analyzed the correlations between different fitness parameters. In the 1st generation of the PxP population, survival rate of immature stage was 17%, while the survival rate of PxR was 68%; the duration of the 4th larval instar (5.30 d) and mortality (25%) of this generation were significantly longer (2.8 d) and higher (1%) than that of PxR, respectively (both p < .001). Upon long-term acclimation, the PxP fitness improved significantly, especially that the survival rate of immature stages increased to approximately 60% in the 15th, 30th, and 45th generations. However, PxP feeding on pea exhibited poorer fitness with longer larval developmental time, shorter total life span, lighter pupa, and lower fecundity in different generations compared with PxP feeding on radish. PxP feeding on pea also showed a significantly lower intrinsic rate of increase (r), net reproduction rate (R 0), finite increase rate (λ), and longer mean generation time (T) than PxP feeding on radish in all generations tested. Significant positive correlations were observed between pupal weight and female fecundity in pea-fed populations, and between female longevity and female fecundity in pea-fed and radish-fed populations. Our findings suggest that P. xylostella adaptation to pea does not improve overall fitness compared with the original host radish, making pea a marginal host for P. xylostella.

Genome-wide analysis of diamondback moth, Plutella xylostella L., from Brassica crops and wild host plants reveals no genetic structure in Australia

Molecular studies of population structure can reveal insight into the movement patterns of mobile insect pests in agricultural landscapes. The diamondback moth, Plutella xylostella L., a destructive pest of Brassica vegetable and oilseed crops worldwide, seasonally colonizes winter canola crops in southern Australia from alternative host plant sources. To investigate movement, we collected 59 P. xylostella populations from canola crops, Brassica vegetable and forage crops and brassicaceous wild host plants throughout southern Australia in 2014 and 2015 and genotyped 833 individuals using RAD-seq for genome-wide analysis. Despite a geographic sampling scale > 3,000 km and a statistically powerful set of 1,032 SNP markers, there was no genetic differentiation among P. xylostella populations irrespective of geographic location, host plant or sampling year, and no evidence for isolation-by-distance. Hierarchical STRUCTURE analysis at K = 2–5 showed nearly uniform ancestry in both years. Cluster analysis showed divergence of a small number of individuals at several locations, possibly reflecting an artefact of sampling related individuals. It is likely that genetic homogeneity within Australian P. xylostella largely reflects the recent colonization history of this species but is maintained through some level of present gene flow. Use of genome-wide neutral markers was uninformative for revealing the seasonal movements of P. xylostella within Australia, but may provide more insight in other global regions where the species has higher genetic diversity.

Cryptic Plutella species show deep divergence despite the capacity to hybridize

Background

Understanding genomic and phenotypic diversity among cryptic pest taxa has important implications for the management of pests and diseases. The diamondback moth, Plutella xylostella L., has been intensively studied due to its ability to evolve insecticide resistance and status as the world’s most destructive pest of brassicaceous crops. The surprise discovery of a cryptic species endemic to Australia, Plutella australiana Landry & Hebert, raised questions regarding the distribution, ecological traits and pest status of the two species, the capacity for gene flow and whether specific management was required. Here, we collected Plutella from wild and cultivated brassicaceous plants from 75 locations throughout Australia and screened 1447 individuals to identify mtDNA lineages and Wolbachia infections. We genotyped genome-wide SNP markers using RADseq in coexisting populations of each species. In addition, we assessed reproductive compatibility in crossing experiments and insecticide susceptibility phenotypes using bioassays.

Results

The two Plutella species coexisted on wild brassicas and canola crops, but only 10% of Plutella individuals were P. australiana. This species was not found on commercial Brassica vegetable crops, which are routinely sprayed with insecticides. Bioassays found that P. australiana was 19-306 fold more susceptible to four commonly-used insecticides than P. xylostella. Laboratory crosses revealed that reproductive isolation was incomplete but directionally asymmetric between the species. However, genome-wide nuclear SNPs revealed striking differences in genetic diversity and strong population structure between coexisting wild populations of each species. Nuclear diversity was 1.5-fold higher in P. australiana, yet both species showed limited variation in mtDNA. Infection with a single Wolbachia subgroup B strain was fixed in P. australiana, suggesting that a selective sweep contributed to low mtDNA diversity, while a subgroup A strain infected just 1.5% of P. xylostella.

Conclusions

Despite sympatric distributions and the capacity to hybridize, strong genomic and phenotypic divergence exists between these Plutella species that is consistent with contrasting colonization histories and reproductive isolation after secondary contact. Although P. australiana is a potential pest of brassicaceous crops, it is of secondary importance to P. xylostella.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1183-4) contains supplementary material, which is available to authorized users.

Monitoring of the Apple Fruit Moth: Detection of Genetic Variation and Structure Applying a Novel Multiplex Set of 19 STR Markers

The apple fruit moth Argyresthia conjugella (Lepidoptera, Yponomeutidae) is a seed predator of rowan (Sorbus aucuparia) and is distributed in Europe and Asia. In Fennoscandia (Finland, Norway and Sweden), rowan fruit production is low every 2–4 years, and apple (Malus domestica) functions as an alternative host, resulting in economic loss in apple crops in inter-mast years. We have used Illumina MiSeq sequencing to identify a set of 19 novel tetra-nucleotide short tandem repeats (STRs) in Argyresthia conjugella. Such motifs are recommended for genetic monitoring, which may help to determine the eco-evolutionary processes acting on this pest insect. The 19 STRs were optimized and amplified into five multiplex PCR reactions. We tested individuals collected from Norway and Sweden (n = 64), and detected very high genetic variation (average 13.6 alleles, He = 0.75) compared to most other Lepidoptera species studied so far. Spatial genetic differentiation was low and gene flow was high in the test populations, although two non-spatial clusters could be detected. We conclude that this set of genetic markers may be a useful resource for population genetic monitoring of this economical important insect species.

Spread and global population structure of the diamondback moth Plutella xylostella (Lepidoptera: Plutellidae) and its larval parasitoids Diadegma semiclausum and Diadegma fenestrale (Hymenoptera: Ichneumonidae) based on mtDNA

The diamondback moth (DBM) (Plutella xylostella) is one of the main pests of brassicaceous crops worldwide and shows resistance against a wide range of synthetic insecticides incurring millions of dollars in control costs every year. The DBM is a prime example of the introduction of an exotic species as a consequence of globalization. In this study we analyzed the genetic population structure of the DBM and two of its parasitic wasps, Diadegma semiclausum and Diadegma fenestrale, based on mitochondrial DNA sequences. We analyzed DBM samples from 13 regions worldwide (n = 278), and samples of the two wasp species from six European and African countries (n = 131), in an attempt to reconstruct the geographic origin and phylogeography of the DBM and its two parasitic wasps. We found high variability in COI sequences in the diamondback moth. Haplotype analysis showed three distinct genetic clusters, one of which could represent a cryptic species. Mismatch analysis confirmed the hypothesized recent spread of diamondback moths in North America, Australia and New Zealand. The highest genetic variability was found in African DBM samples. Our data corroborate prior claims of Africa as the most probable origin of the species but cannot preclude Asia as an alternative. No genetic variability was found in the two Diadegma species. The lack of variability in both wasp species suggests a very recent spread of bottlenecked populations, possibly facilitated by their use as biocontrol agents. Our data thus also contain no signals of host-parasitoid co-evolution.

Reliability of Degree-Day Models to Predict the Development Time of Plutella xylostella (L.) under Field Conditions

The diamondback moth, Plutella xylostella (L.), is a cosmopolitan pest of brassicaceous crops occurring in regions with highly distinct climate conditions. Several studies have investigated the relationship between temperature and P. xylostella development rate, providing degree-day models for populations from different geographical regions. However, there are no data available to date to demonstrate the suitability of such models to make reliable projections on the development time for this species in field conditions. In the present study, 19 models available in the literature were tested regarding their ability to accurately predict the development time of two cohorts of P. xylostella under field conditions. Only 11 out of the 19 models tested accurately predicted the development time for the first cohort of P. xylostella, but only seven for the second cohort. Five models correctly predicted the development time for both cohorts evaluated. Our data demonstrate that the accuracy of the models available for P. xylostella varies widely and therefore should be used with caution for pest management purposes.

Plutella australiana (Lepidoptera, Plutellidae), an overlooked diamondback moth revealed by DNA barcodes

The genus Plutella was thought to be represented in Australia by a single introduced species, Plutella xylostella (Linnaeus), the diamondback moth. Its status as a major pest of cruciferous crops, and the difficulty in developing control strategies has motivated broad-ranging studies on its biology. Prior genetic work has generally supported the conclusion that populations of this migratory species are connected by substantial gene flow. However, the present study reveals the presence of two genetically divergent lineages of this taxonin Australia. One shows close genetic and morphological similarity with the nearly cosmopolitan Plutella xylostella. The second lineage possesses a similar external morphology, but marked sequence divergence in the barcode region of the cytochrome c oxidase I gene, coupled with clear differences in genitalia. As a consequence, members of this lineage are described as a new species, Plutella australiana Landry & Hebert, which is broadly distributed in the eastern half of Australia.

Ecology of diamondback moth in Australian canola: landscape perspectives and the implications for management

The ecology of diamondback moth (DBM), Plutella xylsotella L. (Lepidoptera: Plutellidae), and records of its frequent, but sporadic, population outbreaks in the canola agroecosystems of southern and western Australia are reviewed. The migratory capacity of DBM, possible maintenance of pest populations on brassicaceous weeds and forage crops, resistance to commonly used pyrethroid insecticides, a lack of effective natural enemies (due to disruption by insecticides and difficulties associated with colonising the vast areas of canola crops) and suitable climatic conditions during critical phases of the crop cycle are all likely to contribute to the observed pest outbreaks. A greater understanding of the ecology of DBM in the canola landscape is fundamental to improving its management in the crop but relevant long-term DBM abundance data are currently lacking. Five critical research issues are identified: (i) improved understanding of the factors which determine regional movement patterns of diamondback in canola-growing areas; (ii) the development and implementation of flexible insecticide resistance management strategies; (iii) better understanding of canola crop colonisation by natural enemies of DBM and their population dynamics under current and alternative insecticide application strategies; (iv) greater appreciation of the interactions between DBM and its crop and weedy host plants; and (v) the development of validated simulation models to aid in the forecasting of possible DBM outbreaks. Each issue represents a significant challenge but all must be addressed if the development of a sustainable integrated strategy for the management of DBM in Australian canola is to become a reality.

ISSR-PCR: tool for discrimination and genetic structure analysis of Plutella xylostella populations native to different geographical areas

The diamondback moth (DBM), Plutella xylostella (L.) is considered as the most destructive pest of Brassicaceae crops world-wide. Its migratory capacities and development of insecticide resistance in many populations leads to more difficulties for population management. To control movement of populations and apparitions of resistance carried by resistant migrant individuals, populations must be identified using genetic markers. Here, seven different ISSR markers have been tested as a tool for population discrimination and genetic variations among 19 DBM populations from Canada, USA, Brazil, Martinique Island, France, Romania, Austria, Uzbekistan, Egypt, Benin, South Africa, Réunion Island, Hong Kong, Laos, Japan and four localities in Australia were assessed. Two classification methods were tested and compared: a common method of genetic distance analyses and a novel method based on an advanced statistical method of the Artificial Neural Networks' family, the Self-Organizing Map (SOM). The 188 loci selected revealed a very high variability between populations with a total polymorphism of 100% and a global coefficient of gene differentiation estimated by the Nei's index (Gst) of 0.238. Nevertheless, the largest part of variability was expressed among individuals within populations (AMOVA: 73.71% and mean polymorphism of 94% within populations). Genetic differentiation among the DBM populations did not reflect geographical distances between them. The two classification methods have given excellent results with less than 1.3% of misclassified individuals. The origin of the high genetic differentiation and efficiency of the two classification methods are discussed.